Improv comedy classes helped me a lot. Not even necessarily with spontaneity. They helped me build confidence. "I don't know what this situation is, and that feels yucky. But I've felt that kind of yuck a lot of times before so I'm pretty sure I'll be able to navigate it."

One of my improv instructors gave me this advice about conversations that always resonated with me: just make the conversation about the other person. If you get to the end of it, ask a question about them.

"Are you traveling for the holidays?" "See any movies lately?" "Where'd you get that ring?" "How are your cats doing?"

Anything, as long as it's about them. Keep them talking. A metaphor we learned was "bull and matador." You're dealing with someone who has their own full, rich inner life. They're like a bull ready to lunge forward with a new spike of conversation. Sometimes you just need to wave your cape this way or that to get them to do the work.